Obtaining a Lunar Abyssite is just like adding an empty equipment slot. You'll still need to obtain Atma key items in order to put anything into that slot. The Atma Infusionist basically lets you equip something in that slot, and the Atma Fabricant rewards you with special Atmas based on clearing missions, defeating certain NMs outside of Abyssea, or clearing other older in-game content. The Atma Fabricant can give you different Atmas depending on which zone you're in, so check this list to see what you're eligible for: http://wiki.ffxiclopedia.org/wiki/Atma_Fabricant . Most of the powerful Atmas have to be obtained by killing Abyssea NMs (better chances if you trigger a red !! weakness using elemental WS's, so bring a WAR or NIN friend if possible) or by completing Abyssea quests. The full list of Atmas can be found here: http://wiki.ffxiclopedia.org/wiki/Atma